The present invention relates generally to electrographic marking devices, and more particularly, to a writing head, also referred to as a print head or a recording head, for producing latent electrostatic charge patterns on an insulating medium to form a visible image.
The electrographic marking process for producing a single-color visible image can be generally and briefly characterized as a two step process: The first step involves forming an electrostatic latent image on a medium at a writing station using a writing head. The second step involves rendering visible the electrostatic latent image that is deposited on the medium by toning or developing the latent image using a liquid or dry toner in the selected color. In the case of dry toner, some type of fusing of the toner to the medium may be employed, as, for example, a process known as flash fusing. In the case of liquid toner or ink, provision may be made to aid in the removal of excess ink followed by the drying of the liquid toned medium surface. The result in either case is a permanent and fixed single-color image formed on the medium. The forming of full color electrostatic images generally involves depositing latent images of the color separations that comprise the full color image in registration with each other on the medium; while there may be considerable complexity in registering the multiple latent images, the writing of each individual image generally follows the two-step process just discussed. An example of a color electrographic image-forming apparatus and a method for forming a full color image using the device are described in U.S. Pat. No. 4,569,584.
In an electrographic marking device, an exemplary writing head comprises a plurality of writing electrodes physically positioned to electrically address a dielectric surface of the medium as the medium travels through the writing station. An aligned series of backup electrodes is positioned opposite to the writing electrodes of the writing head in a manner that leaves a small gap, and the medium on which the image is to be formed passes through this gap. When the potential difference between the addressed writing electrodes and the opposed backup electrodes is raised to a threshold level of several hundreds of volts, referred to as the Paschen breakdown point, an electrostatic charge is deposited on the dielectric portion of the medium as that medium is moved through the gap. The timing and sequencing of energization of the electrodes provides for electrical charging of selected areas of the medium to form a desired latent image as the medium is moved through the writing station.
When the image to be formed on the medium is considered to be structured as a two-dimensional array of rows and columns of image spots, the latent image is typically formed row by row (or column by column), requiring the writing head to contain a writing electrode, referred to as a "nib" herein, for each spot to be formed in a row (or column) of the image. Thus, the writing head must be as wide as the visible image desired, which is typically related to the width of the medium, and the nibs must be as closely spaced as necessary to form a visible image having the desired resolution. The closely spaced nibs, however, must be able to be independently electrically controlled, requiring a suitable electrical connection from each nib to circuitry that controls the formation of the image. The line of closely spaced nibs will be referred to herein as the "nib line" of the writing head, and each row of the latent image produced by the nib line will be referred to as a "scan line" of the image.
U.S. Pat. No. 3,693,185 issued in 1972 to Lloyd discloses an electrostatic writing head that comprises first and second series of conductors disposed in spaced, parallel relation and having a pair of elongated, insulative head members secured together in confronting relation that sandwich one end of each of the conductors therebetween. The tips of the conductor ends positioned between the head members are the writing electrodes or nibs, and are exposed in a line lying substantially in a plane between the insulative head members; this line of nibs forms the nib line of the writing head. The writing head disclosed in U.S. Pat. No. 3,693,185 further includes first and second elongated handling elements that are readily and releasably secured to each of the other end of the first and second series of conductors, respectively, so that the ends of the conductors to be connected to the drive circuitry can be readily peeled from these elongated handling elements and soldered to a printed circuit board, for example, to make the appropriate electrical connections. Lloyd discloses that the elongated handling elements permit the wires to be handled so as to minimize their entanglement with one another.
U.S. Pat. Nos. 3,693,185 and 3,793,107 (hereafter also referred to as the Lloyd patents) disclose a method of construction of this writing head that involves winding a length of wire about a mandrel to form uniformly laterally spaced convolutions of wire. An elongated strip of insulative material that is coated with a hardenable adhesive material extends transversely to the plane of the uniformly spaced wire convolutions and is positioned beneath them. This coated strip is then moved radially outwardly of the convolutions to urge the adhesive material against portions of the convolutions. Then another strip of insulative material is adhered to the convolutions in confronting relation to the first named strip so as to sandwich portions of the convolutions between the confronting strips. The strips are then compressed tightly together and after permitting the adhesive material to harden, the convolutions and strips are severed by cutting through both the strips and the convolutions along a line extending lengthwise of the strips; each of the two lines of wire tips exposed by the cutting operation forms the nib line of a writing head.
FIG. 35 illustrates the apparatus for making the writing head as just described, and FIG. 36 illustrates four writing heads produced as a result of this process, after cutting through both strips 13 and 14 and the convolutions of wire along a line 15 extending lengthwise of the strips. Individual nibs 12 become exposed as a result of the cutting process. FIG. 36 also shows elongated handling elements 49, 56, 51 and 57 that protect the other ends of the conductors. It has been estimated that constructing writing heads according to the process disclosed in U.S. Pat. No. 3,693,185 and variations thereof takes an average of 5.5 hours to complete the wire winding process alone and requires eleven (11) miles of wire for a single winding, which for wide writing heads (e.g., 54 inches) results in the production of only one nib line.
In one implementation of an electrostatic writing head similar to the one disclosed in U.S. Pat. No. 3,693,185, the ends of the conductors to be connected to the electronics circuitry that will drive the nibs are connected in the manner shown in FIG. 37, where multiple nibs 136, 138, 140 and 142 of nib line 130 are connected to a single driver 134 on high voltage driver board 132. This type of connection is a multiplexed connection, where a single driver drives more than one nib. It can be seen that this connection process involves manipulating the connector ends of the nibs in a type of weaving process, where wire conductors are threaded across other wire conductors in order to be soldered to the appropriate driver. The weaving process is currently an entirely manual process requiring skilled labor and approximately sixty (60) hours of weaving to complete a nib line. There may be an additional two to as much as ten hours of corrective weaving work after heads are tested and found to fail; much of this reworking involves correcting wires that have been connected to the wrong drivers.
Writing heads may be made in a variety of widths using this process, which provides flexibility for producing multiple smaller-width writing heads from a single winding. For wide image marking requirements, however, such as for devices that support engineering, architecture and graphic arts applications, the writing heads are typically made in single, full-width units. The maximum width of a nib line is subject to the capabilities of the winding apparatus, and considerable retooling of equipment would be required to enlarge the width. Moreover, the wider the writing head desired or the higher the pitch of the writing head, the longer and more costly is the manual weaving process required to connect the wires to the high voltage driver boards.
In addition, it is readily apparent that a writing head made according to this existing process is a bulky and cumbersome component of the electrostatic marking device, with literally thousands of strands of fine gauge wire that are directly connected to the driver board circuitry and consequently need to be carefully protected from damage. Protection of these wires is typically accomplished at various stages of the writing head construction process through the use and application of various types of paper- and fabric-based tapes that function to hold the wires in place and to protect them from breaking during the steps of construction. For example, during the weaving process tapes are used as a protective covering around the wires between the head members that secure the nib line and the driver boards. The tapes are manually applied and removed, and are an added expense in the construction of each writing head.
Still another disadvantage of the existing method for constructing a writing head is the ability to control the placement of the wires that form the nib line with the winding device during winding of the wires. The wires that form the nib line must be laid down during the winding in a manner that maintains a precise inter-wire spacing requirement that is related to the resolution, or pitch, of the desired image, and the size of the toner or ink spot that is deposited on the medium. This requires that a sufficient but not excessive amount of tension be applied to the wire during the winding process to maintain the correct inter-wire spacing between the wire without breaking it. As the desired image resolution increases, the wire becomes finer and finer and is thus more susceptible to breakage, thus reducing the yield of writing heads that may be produced from the process described in U.S. Pat. No. 3,693,185.
Thus it is apparent that there are several disadvantages to producing writing heads according to the process disclosed in U.S. Pat. Nos. 3,693,185 and 3,793,107 and having the structural configuration shown therein.